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Full text of "Text Book Of Mechanical Engineering"

738

Balanced Hydraulic Lift.



paratively small engine, working all the time, may serve for very
heavy work occupying only a short period (see Case 4 especially).
It is in the great storing capacity, and the little loss (skin friction
being independent of pressure, and water incompressible) that
hydraulic transmission is of such immense advantage. The usual
large pressure, 750 Ibs. per sq. in., is adopted because the friction
is then much less in proportion to power transmitted, area of pipe
being small. Chapter VII. illustrates hydraulic transmission
applied to Case 2, and the student may now refer to pp. 292-3,
301-2, 314, 317, 320, and to Plates XV. and XVI., also to
Case 12, p. 580.

Fig. 727 shews Mr. TweddelPs Differential Accumulator,
where great pressure is obtained by considerably decreasing the
ram area. B is the load, and the effective area of ram is A
minus a. Comparing with Fig. 726, it must be understood that,
weights being equal, we lose in time what we gain in pressure, and
thus this apparatus is specially suitable for small machines, such
as portable riveters. The work stored in any accumulator is the
weight or load, in Ibs. x the height lifted, infect, or
w H foot pounds.

A Hydraulic Lift, as devised by Mr. Ellington, and known
as a 'balanced' lift, is shewn in. Fig. 728. A long ram A,
working in a cylinder c, thereby lifts a cage B, and the load .
consists of (i) the cage, (2) the people or goods, and (3) the
ram weight, the last., two being variable. In the older and
dangerous method the average load was balanced by a weight
hung from a cord carried over a pulley, and connected to the top
of the cage; but here the cage and people are lifted by separate
water columns, while the varying ram weight is supported by a
head which similarly varies. The variation in ram weight is due
to the ram's varying immersion, the upward support from the
water (apart from artificial pressure) being equal to the weight of
fluid displaced. Referring to Fig. 728, the pressure from the
main is led to the cylinders r> and E. Upon piston F is a constant
pressure, through L, supporting weight of cage + ram when down ;
and on piston G, through K, pressure water is admitted when
required, supporting the people + friction, viz., the nett load.
Both these pressures are used to intensify the water in M, which